Image sensor chips are widely used in applications such as cameras. In the formation of image sensor chips, image sensors (such as photodiode devices) are formed on a silicon substrate of a wafer, followed by the formation of an interconnect structure on a front side of the wafer. After the formation of the interconnect structure, the wafer is thinned, and backside structures such as color filters and micro-lenses are formed on the backside of the wafer. When the image sensor chips are used, light is projected on the image sensors, in which the light is converted into electrical signals. For example, the image sensors in the image sensor chips generate electrical signals in response to the stimulation of photons.
In the image sensor chips, deep trenches are formed in the silicon substrate to separate the image sensors from each other. The deep trenches are filled with dielectric materials, which may include an oxide, to isolate the neighboring devices from each other.